1. Field of the Invention
The present invention relates to a mount on which substrate to be treated is mounted, and a plasma processing apparatus in which the mount is used. More particularly, it relates to an improvement of the mount on which a semiconductor wafer is mounted.
2. Description of the Related Art
In the case of the semiconductor processing apparatus, a sheet of the semiconductor wafer is mounted on a conductive mount, which is called susceptor, in a vacuum reactor chamber and a certain process is applied to the surface of the wafer. Recently, those of the electrostatic chuck type are being used as the conductive mount or susceptor. According to these mount, the wafer is attracted and held on the susceptor by electrostatic power without using any mechanical holder means such as the cramp.
In the case of the initial electrostatic chucks of this type, the surface of the susceptor is oxidized to form an insulating film on it. High voltage is applied to the susceptor and the insulating film on the susceptor surface is polarized. Static electricity is thus caused on this susceptor surface and the wafer is attracted and held on the susceptor by the coulomb force of this static electricity. In the case of these susceptors of the electrostatic chuck type, however, the insulating film on the susceptor surface is not fully polarized and electrostatic attraction force thus created is not enough to hold the wafer on the susceptor. In order to overcome this, a susceptor which is covered by an electrostatic chuck film is now mainly being used.
FIG. 1 shows this wafer mount which is used in the plasma etching apparatus of the type processing wafers one by one and which is of the electrostatic chuck type. A focus ring 102 is arranged around a susceptor 100 to apply uniform process to the face of a semiconductor wafer 102 and an electrostatic chuck sheet 106 is arranged on the top of the susceptor 100.
The electrostatic chuck sheet 106 comprises a pair of polyimide resin films 108 and 110 and a thin conductive film 112 such as a copper foil sealed between the polyimide resin films. The sheet 106 is formed as a flat disk to match the top (or wafer-supporing face) of the susceptor 100. It is provided with a plurality of through-holes (which correspond to holes identified by reference numeral 20e in FIG. 5) and inactive gas such as He is supplied from the susceptor 100 to and between the wafer 102 and the sheet 106 through the holes to improve thermal conductibility between them.
The conductive film 112 of the sheet 106 has a diameter smaller than those of the polyimide resin films 108 and 110 which are bonded to each other at a rim section 106a of the sheet 106. In other words, this rim section 106a corresponds to a overlap width of the resin films 108 and 110. The rim section 106a inevitably results from the structure of the electro-static chuck sheet in which the conductive film is sealed between the two resin films. However, the insulating function of preventing the conductive film 112 from being exposed and shortened with external components (particularly with the susceptor) can be achieved by the rim section 106a.
Diameters of the top of the susceptor 100 and the sheet 106 are set smaller than that of the wafer 102, as shown in FIG. 1. This is intended to allow reactive ions to enter not into the wafer-supporting surface of the susceptor but only into the surface of the wafer 102. When reactive ions in plasma enter into the wafer-supporting surface of the susceptor 100 in the case of the plasma etching process, for example, that portion of the wafer-supporting surface into which reactive ions have entered is etched. This is not preferable.
In the case of the mount having the above-described arrangement, however, the following problems are caused.
The diameter of the wafer-supporting surface of the susceptor 100 is made smaller than that of the wafer 102. Therefore, the marginal portion of the wafer 102 is a little projected outside from the brink of the susceptor 100 only by a distance (ra). In addition to this portion (ra), that portion of the wafer 102 which is contacted with the rim 106a of the sheet 106 is also projected from the end portion of the conductive film 112 in the sheet 106 by a distance (rb). Therefore, attraction force applied from the conductive film 112 in the sheet 106 to the wafer 102 is reduced at that marginal portion of the wafer 102 which extends over a distance (R) represented by the sum of distances (ra) and (rb). Thermal conductibility between the sheet 106 and the marginal portion of the wafer 102 is thus lowered to thereby reduce the capacity of uniformly heating the wafer 102. However, the rim or margin 106a of the sheet 106 is inevitably and technically needed from the viewpoint of the sheet making and the shortening stop. The sheet 106 therefore has a limit in that its rim or margin 106a is made smaller.
The focus ring 104 encloses the susceptor with a clearance 114 of 0.1-0.3 mm interposed between them so as to protect the susceptor 100 from plasma. When voltage applied to the susceptor 100 or the concentration of plasma is quite high, however, discharge is caused from the susceptor brink or corner 116 to thereby damage or break the susceptor 100 and the electrostatic chuck sheet 106.